1. The Field of the Invention
The present invention relates to apparatus and methods for measuring milk rigidity in the manufacture of fermented dairy products. More particularly, the present invention is directed to apparatus and methods for measuring the coagulation time of and the degree of milk rigidity in the manufacture of fermented dairy products such as cheese and for determining when to cut the curd formed by the coagulated milk.
Although the present invention relates to apparatus and methods for measuring milk rigidity in the manufacture of many different types of fermented dairy products, the following discussion of the present invention, as well as the discussion of the prior art, is generally in terms of cheesemaking. However, since the processes for making other fermented dairy products are closely akin to the processes for cheesemaking, it will be readily understood that the description of the present invention also pertains to the manufacture of other fermented dairy products.
2. The Prior Art
Cheese is made by the controlled coagulation and syneresis of milk. Each year, the cheesemaking industry in the United States consumes literally billions of gallons of milk for the production of cheese. Coagulation and syneresis of the milk is accomplished by an extract containing an enzyme known as rennin or chymosin, which enzyme is extracted from the fourth or true stomach of a calf. (Other suitable enzyme-containing extracts obtained from bovine, swing, and fungal sources are also used.)
Upon action by the rennin, the milk is converted into a cheese curd and whey. The activity of the rennin is enhanced or catalyzed by both heat and lactic acid. Generally, lactic acid is supplied by lactic acid-producing bacteria, such as Streptococcus lactis and Streptococcus cremoris. Such bacteria feed primarily on the lactose in milk to produce the acid needed in the manufacture of cheese.
A bulk culture of lactic acid-producing bacteria is typically prepared in a vessel known as a bulk culture tank and serves as an inoculant for the milk to be made into cheese. This bulk culture of lactic bacteria generally comprises from about 0.1% to about 5% or more of the total volume of milk to be inoculated.
Once a satisfactory lactic bacteria bulk culture has been prepared in the bulk culture tank, the bulk culture is introduced into a cheesemaking vessel containing the milk. The rennin enzyme is also added to the milk in the cheesemaking vessel, and the lactic bacteria cultures produce the necessary acid to aid the enzyme in producing cheese.
In the manufacture of fermented dairy products such as cheese, it is important to monitor the progressive coagulation of the milk in order to determine when the formed curd should be cut. One test for determining when the curd should be cut involves the insertion of a dairy thermometer into the curd at about a 45.degree. angle. The thermometer is then lifted straight up out of the curd; if a clean split of the curd results, the curd strength or tension has developed to the desired point for cutting the curd. This curd is typically cut by pulling cutting wires or bars through the curd.
The timing involved in the cutting of the curd is important in terms of obtaining the maximum cheese yield. For example, if the curd is cut too soon, many of the cheese-forming solids may be lost in the whey, resulting in less product. If the curd is cut too late, it may be difficult to pull the cutting wires or bars through the curd to form uniform curd particles. Moreover, if the curd is cut too late, the resulting curd particles are so firm that it takes a relatively long period of time for the whey to be released from the curd particles. The result is that a substantially longer period of time is required to obtain a curd product of the desired moisture content.
A variety of factors are involved in determining the exact coagulation rate of the milk; e.g., the composition of the milk used to make the cheese, the activity of the lactic bacteria bulk culture, the particular enzyme used to coagulate the milk, the temperature within the cheesemaking vessel, the salt concentration of the milk and lactic bacteria bulk culture, and the previous enzymatic activity in the milk. These numerous variables make it difficult to accurately predict the coagulation rate, and thus the proper time at which the curd should be cut.
After the curd has been cut, the cut curd is generally allowed to settle and heal until the freshly cut curd surfaces harden slightly. Thereafter, the cut curd is then agitated to facilitate whey removal and prevent curd particle fusion until the desired cheese moisture is obtained. By allowing the curd surfaces to harden during healing of the curd, shattering of the curd particles is reduced when the cut curd is agitated.
As discussed below, many different apparatus have been developed for measuring milk coagulation time to help predict the proper time for cutting the curd in the manufacture of fermented dairy products such as cheese. However, these prior art devices have not been commercially accepted, and most cheese producers cut the curd after a set period of time, whether or not it is the ideal time for cutting the curd. The result is often wasteful. While there are some devices for determining the time to cut the curd, there has not, unfortunately, been any adequate instrumentation which can determine when proper healing of the curd has occurred so that agitation can begin. Again, most cheesemakers use an "educated guess" at this time.
In one device developed to measure milk coagulation time, the degree of coagulation is estimated by measuring the drip rate of the coagulating milk through a capillary tube at various time intervals. (See, e.g., G. Scott Blair et al., "A Simple Method for Detecting an Early Stage in Coagulation of Rennetted Milk," 30 J. Dairy Res. 383-390 (1963).) In another apparatus, sound waves are passed through the milk as it coagulates and the increase in the velocity of the sound waves is measured as an indication of the extent of coagulation. (See, e.g., abstract M73 from 51 J. Dairy Sci. 940 (No. 6), T. Everson et al., "Rennet Coagulation Test with a Recorded Endpoint.")
Milk coagulation monitoring apparatus have also been developed wherein the drag on an object pulled through the coagulating milk is measured or wherein the resistance to an oscillating wire within the coagulating milk is measured to indicate the extent of coagulation. (See, e.g., D. McMahon et al., "Evaluation of Formagraph for Comparing Rennet Solutions," 65 J. Dairy Sci. 1639-1642 (1982).)
Certain apparatus employing diaphragms have been developed which measure the resistance of the coagulating milk to oscillatory deformation. (See, e.g., A. Kowalchyk et al., "Firmness of Enzymatically-Formed Milk Gels Measured by Resistance to Oscillatory Deformation," 61 J. Dairy Sci. 1375-1379 (1978).) In one such apparatus, two juxtaposed diaphragms are placed into the coagulating milk sample; one diaphragm (the transmitting diaphragm) oscillates and sends pulses through the coagulating milk to the other diaphragm (the receiving diaphragm). The extent of the pulses or deformations transmitted by the transmitting diaphragm through the coagulating milk to the receiving diaphragm is indicative of the firmness of the milk coagulum.
Other apparatus, typically referred to as torsiometers, involve the use of a cylinder which is suspended into the coagulating milk and oscillated through a fixed angle. (See, e.g., J. Oosthuizen et al., "A Constant Speed, Fixed Angle Torsiometer for Measuring the Coagulation of Milk by Rennet," 9 S. Afr. J. Agric. Sci. 1011-1018 (1966).) The restraining drag on the rotating cylinder is measured as the milk coagulates. The increasingly larger torque force needed for oscillating the cylinder through the fixed angle is indicative of the degree of coagulation.
A thrombelastrograph is yet another device which has been used for measuring the coagulation time of milk. (See, e.g., N. Olson et al., "Rheology of Milk Gels Formed by Milk-Clotting Enzymes," 42 Journal of Food Science 669-673 (1977).) In a typical thrombelastrograph device, two coaxial cylinders are placed within the sample of coagulating milk. The outer cylinder is caused to rotate; thus, as coagulation occurs, the rotational motion of the outer cylinder is transferred to the inner cylinder which is suspended by a wire. A mirror on the wire supporting the inner cylinder in the milk coagulum reflects a light beam to photosensitive recording paper, thereby recording the oscillations of the inner cylinder which are indicative of the degree of coagulation.
Still another apparatus used in the prior art is the vibrating-reed viscometer. (See, e.g., R. Marshall et al., "Assessment of Two Instruments for Continuous Measurement of the Curd-Firming of Renneted Milk," 49 J. Dairy Res. 127-135 (1982).) The vibrating-reed viscometer apparatus measures the voltage which is required to keep a reed vibrating within the milk sample as it coagulates.
Despite the numerous apparatus and methods which have been developed in the prior art for measuring the progressive coagulation of milk in making fermented dairy products such as cheese, several significant problems are still encountered in the prior art apparatus and method. One important limitation of the prior art is that, typically, the prior art apparatus are designed for testing milk samples on a laboratory scale and not on an actual industrial operating scale.
Such laboratory scale apparatus often do not adequately predict the actual coagulation times which will be encountered in industrial operations. For example, the conditions within an industrial scale cheesemaking vat are considerably different from those conditions within the laboratory-sized receptacles which are often used to test the coagulation time of milk in the prior art. Thus, the laboratory measurements provided by such prior art apparatus have often not provided accurate data from which to predict the coagulation time and curd cutting time in making cheese and other fermented dairy products on an industrial scale.
A typical cheesemaking vat holds from about 2300 to about 6400 gallons of milk (representing approximately 20,000-55,000 pounds of milk), and thus, significant movements of these large volumes of milk within the vat are typically experienced during cheesemaking. The prior art apparatus have encountered significant difficulty in making accurate and reproducible measurements within the cheesemaking vat, because the motion of the milk within the vat disturbs the operation of the apparatus. This is one important reason why the prior art has dealt primarily with laboratory scale apparatus.
In addition, any device which is used in the large commercial cheesemaking vats cannot be overly complicated and fragile. Unfortunately, most of the prior art devices are complicated, fragile, and overly sensitive to typical setup procedures and operating conditions.
As a result, the prior art apparatus are typically either limited to laboratory scale application (which experimental results are often difficult to extrapolate to the industrial scale) or do not provide accurate measurements of the milk coagulation when placed directly into large industrial scale, fermented diary product-making vessels, e.g., into a cheesemaking vat. The prior art has generally thus not provided accurate and reproducible coagulation time data for cheese produced on an industrial scale and for other fermented dairy product-making operations.
Another problem encountered in the prior art is that of obtaining a quiescent state for the milk by the time that coagulation begins. Typically, when the lactic bacteria bulk culture and enzyme are added to the milk, the milk is thoroughly agitated so as to mix the bulk culture and enzyme into the milk. As a result, significant movements are experienced within the milk in the cheesemaking vat. Such movement is undesirable during coagulation of the milk since motion disturbs the forming curd and a loss of solids in the formed curd may result from such disturbance. Hence, it is desirable to have minimal motion in the milk during the coagulation of the milk. Baffles or other restriction devices are often installed into the cheesemaking vat in order to obtain quiescence of the milk before significant coagulation has begun. However, the prior art has not provided any method for determining the time elapsed before the milk is rendered quiescent so that it may be determined if such baffles or other restriction devices are needed.
From the foregoing, it will be appreciated that what is needed in the art are apparatus and methods for measuring milk coagulation times and curd firming rates in the manufacture of fermented dairy products (such as cheese), wherein the apparatus and method can be conveniently and simply utilized directly in large scale industrial operations, as well as in laboratory scale experiments.
Additionally, it would be a significant advancement in the art to provide such apparatus and methods which are not significantly affected by the movement of the coagulating milk within the industrial scale, fermented dairy product-making vessel, and which provide accurate and reproducible coagulation time data on both the industrial scale and the laboratory scale.
It would be a further advancement in the art to provide an apparatus and method for determining when adequate healing of the cut curd has occurred and when the cut curd should be agitated. Such apparatus and methods are disclosed and claimed herein.
It would be yet another advancement in the art to provide an apparatus and method for determining when the agitated milk, lactic bacteria bulk culture, and enzyme have become quiescent within a cheesemaking vat so as to determine whether or not mechanical restriction devices are needed to accelerate quiescence.